Monday, 29 September 2014

Flowering plants (angiosperms) began to dominate the Earth’s
vegetation about 100 million years ago and, while other, more primitive, plants
continue to be abundant, the present diversity of angiosperms is remarkable. When
thinking of flowering plants, our minds may turn to beautiful garden borders,
meadows and occasional clumps of flowers in woods and verges. Yet angiosperms have also invaded water bodies; although this is really a re-invasion, as
land plant evolved from distant aquatic ancestors.

Anyone visiting a stream draining from chalk strata is
impressed by the amount of vegetation growing over its bed and invading from
the margins. There are many microscopic algae that are only visible under a
microscope, but two common flowering plants often dominate: water cress and
water crowfoot. Of the two, water cress grows into the stream from the banks
and can extend right across narrow channels, a habit that has been exploited in
the development of commercial cress beds fed by water from chalk streams. The bulk
of the plant remains above the water surface and this contrasts with water
crowfoot, where plants grow in dense stands, rooted into the bed of the stream
and affecting its flow pattern. Water crowfoot is a relative of the buttercup
and its flowers are very similar in structure, although they are white, rather
than yellow, in colour (see below). It is only during flowering that we see water crowfoot
above the water surface, although stands can become so dense that, at times of
low flow in summer, they may be exposed to the air. They are well adapted to
life in flowing water. The drag on the mass of leaves is counteracted by an
effective rhizome and root system that ensures anchorage on the stream bed and
the plants engineer the stream around them. Stands provide an obstruction to
flow that creates channels of faster-moving water between plants and this serves
not only to keep the substratum clear of sediment, but the growing leaves are
also unaffected by deposition and can thus photosynthesise efficiently. In
contrast, the base of the plant is an area of sediment build-up and this
includes much organic matter [1] that serves as a source of nutrients - another
way in which the plants engineer their habitat to their advantage.

Although water cress and water crowfoot are both aquatic
plants, with the former fitting the definition less easily than the latter, seagrasses
are truly aquatic. As their name suggests, these plants are marine, spending
the whole of their life cycle under water. Seagrasses have a world-wide
distribution and are perhaps most commonly associated with tropical seas and,
especially, reefs, where the water is clear and there is good light penetration
to the substratum, allowing efficient photosynthesis. Nutrients needed for
growth are taken up by roots and stored in rhizomes that also serve to stabilise
soft sediments. Interestingly, seagrasses are more closely related to lilies
and ginger than to grasses [2] and their colonisation of soft sediments results
in large "grassy" meadows when conditions for their growth are favourable. These are
then grazed upon by many animals and they also form shelter for many other organisms and a substratum for yet more.

Seagrasses are also found commonly in shallow temperate seas
that have sufficient transparency to allow the plants to grow. As I grew up by
the sea in Torbay, and had a love of Natural History, I knew about seagrasses,
but had no idea that there were meadows of the plants so close to some of my
collecting spots. Nor did I know that seagrasses were flowering plants. Like
many, I thought that seaweeds alone were the dominant large marine plants around
coasts.

Two of my favourite places to visit in Torbay were Elbury
[Elberry] Cove and the rocks below Corbyn’s Head, where I spent time collecting
marine creatures to keep in aquarium tanks. [3] Both locations now have interesting and
informative signs (see below) describing the importance and susceptibility to
damage by boats etc. of the seagrass meadows just offshore.

It is likely that Zostera is one of the seagrasses and Henry
Gosse mentions this plant when describing the results of dredging a little
further up the coast:

Now we have made our offing, and
can look well into Teignmouth Harbour, the bluff point of the Ness some four
miles distant, scarcely definable now against the land. We pull down sails, set
her head for the Orestone Rock [just off Torbay], and drift with the tide. The
dredge is hove overboard, paying out some forty fathoms of line, for we have
about twelve or fourteen fathoms’ water here, with a nice rough, rubbly bottom,
over which, as we hold the line in hand, we feel the iron lip of the dredge
grate and rumble, without catches or jumps. Now and then, for a brief space, it
goes smoothly, and the hand feels nothing; that is when a patch of sand is
crossed, or a bed of zostera, or close-growing sea-weeds, each a good variation
for yielding. [4]

As Gosse was a devout
Creationist, the presence of flowering plants in soft sediments around marine
coasts would be another example of the extraordinary events of the six days in
which all living things - and all fossil ones - came into existence. [5] To
those of us who cannot share such a view, the presence of flowering seagrasses
under water is another example of the extraordinary powers of evolution.

In terrestrial habitats, the fertilisation of ova by pollen
is aided by insects, wind or other agents and there are a diverse range of
adaptations to ensure that fertilisation is achieved - by evolving nectar
and/or scent to attract insects, by evolving elaborate colour patterns that are
attractive, by producing pollen in enormous quantities, etc. - yet flowers are
retained by seagrasses where neither insects or wind can be involved in
pollination. Seagrass plants bear both male and female flowers and the pollen from
male flowers is released into the water and thus wafts around the plants. The
use of water for fertilisation is, of course, extremely common in many marine organisms,
including seaweeds and many animals, and that makes underwater flowers seem
less unlikely than on first consideration. Natural History is full of such
discoveries and one is always learning something new. That’s the satisfaction
of it - that, and the sense of wonder at just what can evolve over millions of
years and millions of generations.

Thursday, 18 September 2014

If an insect falls from a plant it is rarely damaged as it
has a low momentum and its body creates considerable friction drag as it falls.
The force of the impact is reduced as a result and the insect then usually
quickly scurries away.

Some larger animals are also adept at surviving falls and
the offspring of tree-nesting ducks drop to the ground, being cushioned by leaf
litter, by their down-covered bodies and by some flapping of their tiny wings.
[1] Adult birds have little problem as they can fly down and then use sweeping
movements of their wings to produce “reverse thrust” and thus decelerate gently,
using the downward displacement of air to effect an easy landing.

Cats leap from trees, fences and walls, and their landing is
cushioned by the shock-absorbing properties of the limbs, but surviving a fall
from considerable height is much less likely, as maximum acceleration then
results in high momentum. In arboreal mammals, swinging from the arms is a
common feature and, together with the grip of the hands (and feet - and even
tail in some monkeys) ensures that catastrophic falls are avoided. Yet some
animals are able to leap from high in one tree to the base of a nearby tree
while suffering no damage, thus moving more rapidly from one location to
another than could be achieved over the ground. Two well-known examples of
these “fliers” are tree frogs (in the genus Rhacophorus)
and flying squirrels (genus Glaucomys).

Tree frogs spend their adult life in forest canopies, but,
being amphibians, they must find water in which to breed. Some species use small
rain pools in the axils of leaves or in tree holes, while the females of some Rhacophorus build nests of foam attached to
tree branches overhanging ponds, the nest being created by rapid movements of the hind limbs (akin
to whisking) in secretions made by the frog. Eggs are laid within this mass and
these are fertilised by male frogs, hatching tadpoles then emerging into the
foam and dropping into the water below to complete larval life. Froglets leave
the pond and then climb adjacent trees, remaining in the canopy for the rest of
their lives. Although they have pads on their toes to provide excellent
adhesion to surfaces, adult frogs retain the webbed feet of the ancestral forms,
even though they are not used for swimming. The webs are especially
well-developed in some species and they act as parachutes to slow down the
descent when frogs move from one tree to another, or from one branch to a lower
one.

Flying squirrels also use parachutes and these are formed
from loose skin (termed a patagium), that runs between the fore and hind limbs, and between the fore limbs and the head.
I was fortunate to be able to watch Glaucomys
parachuting when visiting Dr Joe Merritt at the Powdermill Biological Station
of the Carnegie Museum of Natural History in Pennsylvania. Dr Merritt had been studying
these mammals for several years and laid out live traps so that we could then
see the squirrels close up. Traps containing animals were collected and each
trap was emptied into a cloth bag. In the photograph below you’ll see me (with
Dr Merritt on the right and the late Prof. Björn Malmqvist on the left) at the moment when a captured squirrel
bit me on the hand. I was told to take a firm grip of the skin over the neck of
the animal, but it was still able to turn its head easily to defend itself from
such unpleasantness. It was a lesson for me in just how much skin that flying
squirrels possess. I let go immediately and the squirrel ran up a nearby tree,
and then made a wonderful gliding flight before climbing rapidly, making
another flight and then disappearing. The parachute of loose skin was very
effective in slowing its descent, the squirrel covering tens of metres and with
good directional control provided by the tail and by changing the profile of the
patagium. It was most impressive and, in one way, I’m pleased that I was
bitten.

I was reminded of this incident when reading a quote from Jeb
Corliss in The Independent:

At the beginning, there were probably
only very few squirrels that even contemplated flying from tree to tree. The
other squirrels thought they were crazy. I imagine that hundreds of them died
in the attempt. But then, in the end, one of them managed it. Now that, to me,
is evolution. And now we are evolving, through technology and through skill. I
liken what we’re doing in proximity flying to the first animals that left the
water. We are evolving and growing. And becoming stronger. What else is the purpose
of life? [2]

Not quite the way I would express the likely evolution of
parachuting in flying squirrels, but Jeb Corliss is an expert wingsuit flyer,
not a Biologist. Of course, it is impossible for humans to control their
descent to the land without an external parachute and the earliest examples
have been transformed into steerable ‘chutes that enable precise landings. As
Jeb points out, wingsuit flying has close similarities to the flight of Glaucomys, with webbing between the
arms, legs and body, analogous to the patagium of flying squirrels, providing
steerable flight. Proximity flying capitalises on this level of control to
allow fliers to pass very close to objects, or the ground, while making their descent
(see the video clip at the end of this post).

Whereas Rhacophorus
and Glaucomys have both evolved
changed body forms to enable them to move from tree to tree for various reasons,
human use of wingsuits is solely for pleasure. The excitement comes from
exposure to danger, a sense of freedom, and the thrill of depending on a skill
where a small mistake can have disastrous results. Some of us are drawn to such
activities and proximity flying is addictive, even though the number of fatalities
is large relative to the number of those who fly. Wingsuit fliers, and others
involved in the most extreme sports, are only too well aware of the dangers and
most are not afraid of death, recognising that it is possible that flights
can go very wrong, even after meticulous preparation. They feel very alive as a
result, and their approach to death contrasts markedly with the fear of life ending that seems
to haunt others within the human population - and which is the basis of many
religions. While animals such as Rhacophorus and Glaucomys cannot be aware of danger in the same way as humans, I wonder
if they get a thrill from flying?

Monday, 8 September 2014

I grew up by the sea and always enjoyed walking on local beaches
looking for shells washed up by the tide. There were many shell fragments, especially
of cockles that must have grown in their millions just offshore, but also a
wide variety of whole shells, especially those of snails. I continued to enjoy
beachcombing and, while on a family holiday in Jersey, found an excellent
beach, dominated by shells of the flat periwinkles Littorina obtusata and L.
mariae. [1] Some were bright yellow, others orange, white, or of a reddish hue.
We collected as many of the shells as we could and, more than twenty years
later, they are still exhibited in a glass jar in our bathroom.

Occasionally, violet shells from the snail Janthina are washed up on beaches, especially
after long periods of strong winds. They are similar in form to those I had collected in Jersey, but they have less strengthening than the Littorina shells, which need to be
strong to withstand the erosive action of the water, and suspended mineral
particles, over the shores on which the snails live. Can we assume, therefore, that
Janthina exists in a less erosive
habitat? Indeed it does - the snails live at the surface of oceans, attached to a
float of bubbles.

In July and August 1954, there were sustained, strong
westerly winds in Great Britain and Ireland and several people reported finding
large numbers of Janthina shells on exposed beaches This prompted Dr Douglas P. Wilson of the Marine Biological
Association to write a letter to The
Times to ask readers for more information about sightings. Reports came
in from many locations and, among all the shells, there were a few living specimens.
These offered the scope for investigations of the formation of the float,
adding to the information acquired by earlier investigators.

Among the first to make observations was Reynell Coates, who,
in 1825, published a description of the float. Coates qualified as a medical
doctor in Philadelphia and then set sail as a ship’s surgeon on a voyage to the
East Indies. He was very interested in Natural History and took samples of
organisms from the surrounding water (presumably when becalmed, or unable to
continue the passage - the journey terminated at Kolkata after the outbreak of
the Burmese War [2]). This is what he wrote about some specimens of Janthina:

Individuals being placed in a
tumbler of brine, and a portion of the float being removed by the scissors, the
animal very soon commenced supplying the deficiency in the following manner:
the foot was advanced upon the remaining vesicles, until about two-thirds of
the member rose above the surface of the water; it was then expanded to the
uttermost, and thrown back upon the water.. ..it was contracted at the edges,
and formed into the shape of a hood, enclosing a globule of air, which was
slowly applied to the extremity of the float. A vibratory movement could now be
perceived throughout the foot, and when it was again thrown back to renew the
process, the globule was found in its newly constructed envelope. [3]

Wilson’s description
of float formation in Janthina included
these observations:

Sometimes the new bubble fails to
be attached and floats away as a tiny glassy sphere.. ..The completed float is firm
between the fingers, springy and dry - it is not in any sense sticky. [4]

Although the bubbles are surrounded by mucus secreted from
glands in the snail’s foot, there are clearly components of this secretion
(proteins?) that, after dehydration of the mucus mass, form a solid coating for
the trapped bubbles. This ensures that the float is near-permanent, although
older pieces break off and need to be replaced. [4] To say that the float is
important for the snail is an understatement as, specimens of Janthina that sink into the water column
cannot regain the surface. [4]

Janthina is a
predator and feeds on other members of the floating community at the air-water
interface. It is especially associated with Velella
(the “by-the-wind sailor”) [3], and both are blown ashore in masses. Velella is a colonial relative of jellyfish,
with polyps attached to a secreted, flattened float that has a sail rising from
it (see below), allowing the colony to be transported by wind. The polyps use
stinging cells to capture small creatures from within the water column, such as
invertebrates and small fish. Wilson quotes Mr Peter David in describing the
manner in which Velella is preyed
upon by Janthina, the snail cutting out
semicircular pieces of the float and attached polyps “in much the same way as a
caterpillar does on the edge of a leaf.” [4]

Gastropod snails, like Littorina
and Janthina, have very similar body
plans and it is relatively easy to see how the latter evolved from a shore-dwelling
ancestor, but how did it develop an ocean-going existence? Observation of pond snails shows that some individuals move across
the underside of the water surface while the foot is held in the surface film and, to these
snails, the interface is like a solid surface. [5] They move here as they do
over the substratum, using muscular contractions of the foot, and feed as they
go. A characteristic of pond snails is their relatively thin shell, as strengthening
calcium salts are not as available in fresh waters as they are in the sea. It
is likely that the ancestral Janthina
did not strengthen the shell in the way that Littorina does and that it moved under surface films as well as
over the substratum, just like some pond snails. In time, the mucus used for
lubrication and attachment during crawling was used to coat bubbles formed by
the foot and this resulted in the development of the float. They then lost the
power of locomotion, or it was highly reduced, and their feeding changed from
scraping materials from surfaces to the removal of sections of prey such as Velella, into which they had drifted, or
which had been blown towards them.

How Velella
evolved its current form is a mystery. There are various theories on how
gastropod molluscs evolved from their ancestral molluscan form, but how did the colony of polyps develop, complete with a float to which
they were attached? Velella has many sedentary colonial relatives
(members of the Cnidaria), but how did life at the ocean surface
begin - and what were the origins of the sail?